The radical high-pressure polymerization of ethylene alone or together with comonomers is performed on a commercial scale in tubular reactors or in autoclaves with agitation at pressures of 50 to 400 MPa, in particular 140 to 250 MPa, and temperatures of 100.degree. to 400.degree. C.
A diagram of the process is shown in FIG. 1. Fresh ethylene is first fed into low-pressure compressor 8 via line 9, compressed to 10 to 35 MPa and then, together with the unreacted ethylene from the polymerization stage and which is recycled, it is raised to the reaction pressure in a high-pressure compressor 5. With the aid of suitable pumping equipment, e.g., piston pumps, the initiators present in solution are mixed into the compressed ethylene or metered into the reactor directly.
Comonomers such as vinyl esters, olefinically unsaturated carboxylic acids, or alpha-olefins are also fed into the high-pressure compressor 5 via feed line 10. In reactor 1 between 10 and 45% by weight of the monomers entering the reactor are reacted in one run. The polymer is precipitated in high-pressure separator 3 by reducing the pressure on the mixture to 5 to 50 MPa, preferably 10 to 35 MPa, after optional cooling in cooler 2. During pressure-relief, the gaseous components released are returned to the high-pressure compressor 5 via high-pressure return gas system 4. The polymer, which contains dissolved unreacted monomers, is pressure-relieved via a system consisting of low-pressure separator 6 and low-pressure cooler 7. The low-molecular weight compounds released are returned via the low-pressure compressor 8 to the high-pressure compressor 5. The polymer precipitated in the low-pressure separator 6 contains not only residual amounts of ethylene and comonomers but also oligomers.
Even when the polymerization products are separated carefully, it is impossible to avoid volatile admixtures such as comonomers, their decomposition products, and other low-molecular weight compounds being retained in the polymers. During storage and processing of the polymers, these residues lead to undesirable emissions and odors; in addition, the flame point of the polymers is considerable reduced. Therefore, efforts are made to insure that the products are free of volatile compounds by taking suitable measures during the pressure-relieving process and by subjecting the polymer to an additional treatment.
Further difficulties are caused by the fact that the ethylene liberated in the low-pressure separator contains comonomers, oligomers, and other compounds which condense out when the separated ethylene is recycled, forming deposits in the compressors and pumps which can lead to clogging.
According to a process described in DD-PS 131 824, either the reaction mixture is brought into contact intensively with the ethylene downstream of the polymerization reactor or the polymer melt is brought into contact intensively with the ethylene after separation of the majority of the unreacted gas mixture in a pressure stage and the mixture is then separated in a stabilizing zone. According to an appropriate embodiment of this process, the reaction mixture is fed countercurrently into a widening tube, ethylene also being simultaneously introduced and the mixture thus formed is separated in the low-pressure separator.
Another procedure for high-pressure polymerization of ethylene is disclosed in the DE-AS 21 31 145. It mixes the reaction mixture coming from the reaction zone with the fresh ethylene feed and the recycled ethylene coming from the low-pressure separator.
Both processes lead only to a slight reduction in the concentration of low-molecular weight admixtures in the polymer. Moreover, additional apparatus is required for their performance.
It is also known that unreacted gas originating from the low-pressure separator can be recycled to the high-pressure cycle with the aid of an injector (cf. DD-PS 202 882). However, this process does not reduce the concentration of the low-molecular components remaining in the product.
With liquid products, the low-molecular weight compounds can be separated by gas scrubbing with nitrogen or other inert gaseous substances. However, additional apparatus is also required for this process and the separated components cannot be returned without cleaning.